The flatiron sketch and the origin of the word flatiron in my sketchbook.

The Flatirons consist of sandstone, sandstone conglomerate or arkose, of the Fountain Formation.  Geologists estimate the age of these rocks as 290 to 296 million years; they were lifted and tilted into their present orientation between 35 and 80 million years ago.  The Flatirons were subsequently exposed by water and wind erosion over the eons.